In the manufacture of tissue products, such as facial tissue, bath tissue, paper towels, dinner napkins and the like, a wide variety of product properties are imparted to the final product through the use of chemical additives. One common attribute imparted to tissue sheets through the use of chemical additives is softness. There are two types of softness that are typically imparted to tissue sheets through the use of chemical additives. The two types are bulk softness and topical or surface softness.
Bulk softness may be achieved by a chemical debonding agent. Such debonding agents are typically quaternary ammonium entities containing long chain alkyl groups. The cationic quaternary ammonium entity allows for the agent to be retained on the cellulose via ionic bonding to anionic groups on the cellulose fibers. The long chain alkyl groups provide softness to the tissue sheet by disrupting fiber-to-fiber hydrogen bonds within the tissue sheet.
Such disruption of fiber-to-fiber bonds provides a two-fold purpose in increasing the softness of the tissue sheet. First, the reduction in hydrogen bonding produces a reduction in tensile strength thereby reducing the stiffness of the tissue sheet. Secondly, the debonded fibers provide a surface nap to the tissue sheet enhancing the “fuzziness” of the tissue sheet. This tissue sheet fuzziness may also be created through use of creping as well, where sufficient interfiber bonds are broken at the outer tissue surface to provide a plethora of free fiber ends on the tissue surface.
A multi-layered tissue structure may be utilized to enhance the softness of the tissue sheet. In this embodiment, a thin layer of strong softwood fibers is used in the center layer to provide the necessary tensile strength for the tissue product. The outer layers of such structures may be composed of the shorter hardwood fibers, which may or may not contain a chemical debonder.
The topical or surface softness of a tissue sheet, and ultimately the resulting tissue product, may be achieved by topically applying an emollient to the surface of the tissue sheet or tissue product. The word emollient is used here in the sense that it makes the tissue sheet less harsh or abrasive. One such emollient is polysiloxane. Polysiloxane treated tissues are described in U.S. Pat. No. 4,950,545, issued on Aug. 21, 1990 to Walter et al.; U.S. Pat. No. 5,227,242, issued on Jul. 13,1993 to Walter et al.; U.S. Pat. No. 5,558,873, issued on Sep. 24, 1996 to Funk et al.; U.S. Pat. No. 6,054,020, issued on Apr. 25, 2000 to Goulet et al.; U.S. Pat. No. 6,231,719, issued on May 15, 2001 to Garvey et al.; and, U.S. Pat. No. 6,432,270, issued on Aug. 13, 2002 to Liu et al., which are incorporated by reference to the extent that they are non-contradictory herewith. A variety of substituted and non-substituted polysiloxanes may be used.
While polysiloxanes may provide improved softness in a tissue sheet and/or tissue product, there may be some drawbacks to their use. First, polysiloxanes are generally hydrophobic, that is, they tend to repel water. Tissue sheets and/or tissue products treated with polysiloxane tend to be less absorbent than tissue sheets and/or tissue products not containing polysiloxanes. For many applications, particularly sanitary bath tissue, this significantly reduces the utility of polysiloxanes to create softness in the tissue sheet and/or tissue product. Hydrophilic polysiloxanes are known in the art, however, such hydrophilic polysiloxanes are typically more water soluble and hence when applied to a tissue sheet and/or tissue product will tend to migrate more in the z-direction of the tissue sheet and/or tissue product than the hydrophobic polysiloxanes. Hydrophilic polysiloxanes typically are also usually sold at a cost premium to the hydrophobic polysiloxanes. The hydrophobic portion of the polysiloxane, referred to as the polydialkylpolysiloxane portion, also tends to have a more significant impact on improving softness. Hence, hydrophilic polysiloxanes also tend to be less effective at softening and more costly to use than hydrophobic polysiloxanes.
An additional disadvantage to the use of polysiloxanes is the effect of aging on hydrophobicity. Elevated temperatures and time may significantly increase the hydrophobicity of treated tissue sheets and/or tissue products and, in cases such as bath tissue, may render the bath tissue product unacceptable for a given application after a certain period of time or under certain environmental conditions.
Polysiloxanes tend to be poorly retained if applied to a slurry of pulp fibers in the wet end of the tissue process. Unretained polysiloxane as well as the additional surfactants required to make emulsions suitable for wet end application can cause significant issues in the tissue making process, rendering wet end application of polysiloxanes infeasible. Furthermore, if applied in the wet end of the tissue making process, hydrophilic polysiloxanes are even more poorly retained on the pulp fibers than the hydrophobic polysiloxanes due to the enhanced water solubility.
For water insoluble polysiloxanes, poor retention of the polysiloxane in the wet end of the tissue machine may be circumvented by treating the pulp fibers at the pulp mill with the polysiloxane prior to final drying of the tissue sheet. Such a process is described in U.S. Pat. No. 6,582,560, issued on Jun. 24, 2003 to Runge, et. al. and which is incorporated by reference to the extent that it is non-contradictory herewith. When used in tissue sheet and/or tissue product, the polysiloxane pretreated pulp fibers may significantly enhance the softness of the tissue sheet and/or tissue product.
Unfortunately, use of these polysiloxane pretreated pulp fibers in tissue sheets or tissue products may lead to unacceptably high levels of hydrophobicity even when low levels of polysiloxane are used. In certain cases, the degree of hydrophobicity introduced into the tissue sheet using polysiloxane pretreated pulp fibers is greater than when the same level of polysiloxane is topically applied to the tissue sheet by the methods known in the art. Additionally, increases in hydrophobicity brought on by heat aging are also present in both the polysiloxane pretreated pulp fibers and/or tissue products made from the polysiloxane pretreated pulp fibers.
Co-pending U.S. patent application Ser. No. 10/289557 filed on Nov. 6, 2002, describes a method to reduce the hydrophobicity associated with use of pulp fibers pretreated with hydrophobic polysiloxanes by altering the layer structure of the tissue sheet. More specifically, by concentrating the polysiloxane fibers towards the exterior surface of the tissue sheet, the hydrophobicity limitations of using pulp fibers pretreated with hydrophobic polysiloxanes in absorbent tissue sheets is overcome. While this method provides a significant improvement in reducing the hydrophobicity of the tissue sheet it does limit use to layered tissue sheets and also generally reduces the amount of total polysiloxane that may be applied. Additionally, while the hydrophobicity is greatly improved, wet out times may still be unacceptably high for bath tissue and similar tissue products.
Co-pending U.S. patent application Ser. No. 10/325484, filed on Dec. 19, 2002, describes a method for applying surfactants topically to the base tissue web to mitigate the hydrophobicity of tissue sheets including tissue sheets prepared with pulp fibers pretreated with hydrophobic polysiloxanes. While this method again improves the hydrophobicity of such tissue sheets, it does require topical application of a surfactant to the tissue sheet and, hence, requires additional capital and added complexity in the tissue making process. An additional disadvantage to using external surfactants is that at least a portion of the surfactants are lost in broke repulping operations. Thus, while the virgin tissue product may be hydrophilic, incorporation of this material into a broke stream could result in tissue products having unacceptable hydrophobicity.
Therefore, there is a need for polysiloxane pretreated pulp fibers that have improved hydrophilic properties while still providing for softness enhancement in tissue sheets and tissue products where the polysiloxane pretreated pulp fibers are incorporated. There is a further need to have the polysiloxane be well retained through the wet end of the paper or tissue making process. There is a further need to be able to utilize such polysiloxane pretreated pulp fibers without the addition of surfactants or other agents to improve the hydrophilicity of the tissue sheets or tissue products made from the polysiloxane pretreated pulp fibers. There is a further need to have the polysiloxane pretreated pulp fibers retain their hydrophilicity when recycled or used in broke and to have the polysiloxane pretreated pulp fibers and tissue sheets or tissue products containing the polysiloxane pretreated pulp fibers exhibit good thermal and aging stability with regard to hydrophobicity.
There is an interest to create polysiloxane pretreated pulp fibers having good retention of polysiloxane through the wet end of the paper making process, provide enhanced softness to products containing the fibers and yet demonstrate improved hydrophilic properties relative to use of hydrophobic polysiloxanes alone. It has now been discovered that pulp fibers pretreated with certain amino functional polyether polysiloxanes may be retained very well through the wet end of the papermaking process despite having excellent hydrophilic properties. Furthermore, such polysiloxane pretreated pulp fibers are effective at improving softness of tissue sheets or tissue products containing the polysiloxane pretreated pulp fibers and providing for enhanced thermal stability relative to hydrophobicity generation. It has also been further discovered that these polysiloxanes may be used in conjunction with hydrophobic polysiloxanes to significantly improve the hydrophilic nature of the pulp fibers and associated tissue sheets or tissue products containing the polysiloxane pretreated pulp fibers.